Hydrocarbon oil generally contains sulfur compounds that, when hydrocarbon oil is used as a fuel, sulfur incorporated therein in sulfur compounds is converted into sulfur oxides and is discharged into the atmosphere. It is preferred, from the viewpoint of reducing air pollution which is caused by combustion of hydrocarbon oil, to reduce the sulfur content of such hydrocarbon oil to a level as low as possible. This can be achieved by subjecting the hydrocarbon oil to a catalytic hydrodesulfurization process (HDP).
Since environmental pollution problems such as acid rain and nitrogen oxides (NO.sub.x) are of great concern worldwide, the removal of the sulfur component from oil at the present technological level is still insufficient. It is, in fact, possible to further reduce the sulfur content of hydrocarbon oil to some extent by operating the HDP under more severe conditions, for example, by controlling the LHSV, temperature, and pressure. HDP under such severe conditions, however, produces carbonaceous deposits on the surface of the catalyst, which, in turn, causes an abrupt drop in catalyst activity. The problem is even worse with a light fraction hydrocarbon oil, since HDP operated under severe conditions adversely influences, for example, on the color hue stability and storage stability of the oil. It can be seen that improvements in operating conditions are only effective to a certain extent, and more substantial improvements must be sought in the development of catalysts considerably increased in catalyst activity.
Hydrodesulfurization catalysts were conventionally produced by methods such as the so-called "impregnation process", which comprises impregnating a carrier with an aqueous solution having dissolved therein a salt of a metal belonging to Group VIII of the Periodic Table (sometimes referred to simply as a "Group VIII metal", hereinafter, and in the same way for a metal belonging to Group VIB of the Periodic Table) and a salt of a Group VIB metal, and, after drying, calcining the metal-impregnated carrier; the so-called "coprecipitation process" which comprises adding an aqueous solution of a salt of a Group VIB metal and an aqueous solution of a salt of a Group VIII metal to an aqueous solution having dispersed therein alumina or a gel thereof to effect coprecipitation of a metal compound; and the so-called "kneading process" which comprises kneading, while heating, a paste mixture composed of alumina or a gel thereof, an aqueous solution containing a salt of a Group VIB metal, and an aqueous solution containing a salt of a Group VIII metal, to remove water therefrom. For reference, see Ozaki, ed., Shokubai Chousei Kagaku (Catalyst Preparation Chemistry), pp. 250 to 252, published by Kodansha Scientific.
None of the aforementioned methods, however, are suitable for uniformly dispersing a relatively large amount of metal compounds in and on the carrier.
With respect to desulfurization processes using a conventional catalyst, for example, the catalytic hydrodesulfurization of a gas oil containing 1.3% by weight of sulfur carried out at a liquid hourly space velocity of 4 hr.sup.-1, at a reaction temperature of 350.degree. C., and at a reaction pressure of 35 kg/cm.sup.2 G, such a process yields, at best, an oil where the sulfur content has been reduced to a value in the range of from about 0.13 to about 0.19% by weight.
As another example, in the case of a vacuum gas oil (VGO) initially containing 2.50% by weight of sulfur, catalytic hydrodesulfurization at a liquid hourly space velocity of 0.4 hr.sup.-1, a reaction temperature of 350.degree. C., and at a reaction pressure of 52 kg/cm.sup.2 G yields a VGO oil the sulfur content of which is reduced only to an insufficient degree, with the limit being in the range of from about 0.15 to about 0.18% by weight.
As a further example, a topped crude having a 3.8% by weight sulfur content obtained from a crude oil results in a product the sulfur content of which is lowered, but the result is limited to the range of from about 0.9 to about 1.0% by weight, after catalytic hydrodesulfurization at a liquid hourly space velocity of 1.0 hr.sup.-1, at a reaction temperature of 361.degree. C. and at a reaction pressure of 150 kg/cm.sup.2 G.
It is desired, therefore, to obtain more readily and without performing the HDP under severe operating conditions, a light gas oil where the sulfur content by weight is reduced to a value in the range of from about 0.05 to about 0.08%, and, similarly, a VGO and a topped crude which have a reduced sulfur content in the range of from about 0.08 to about 0.10% and from about 0.7 to about 0.8%, respectively. If this would be possible, not only would such a process be greatly advantageous from the economical viewpoint in increasing the life of the catalyst but also the resulting oil products would be effective for avoiding air pollution.
An object of the present invention is to provide a catalyst having highly dispersed therein active metals, which catalyst exhibits extremely high desulfurization activity under ordinary operating conditions such that severe process conditions can be excluded.
Another object of the present invention is to provide a fuel oil less apt to cause air pollution due to a reduced amount of sulfur compounds contained therein so that the discharge thereof at combustion may be minimized.